The results of the present study may be briefly summarized as follows. Acid production and sensitivity to methylene blue of 55 strains of methemoglobin-forming streptococci from various sources were studied to determine whether it is possible on the basis of these reactions to differentiate strains of human, bovine, and dairy origin.
Twelve strains were isolated from human infections, 6 from bovine mastitis, and 37 from dairy products. The final hydrogen ion concentrations of all cultures in dextrose broth regardless of the sources from which the strains originated, covered the range pH 5.2 to 4.1. There appeared, therefore, to be no correlation between the amount of acid produced by a given strain and the source from which it was originally derived. Low acid production, a characteristic of hemolytic streptococci of the human type, was not a distinguishing mark of the green producing organisms from human infection, and high acid production was not limited to strains from bovine and dairy sources.
Differences between saprophytic and parasitic strains in their sensitivity to methylene blue in milk were not so sharply defined as were those previously demonstrated for the hemolytic streptococci. Of 18 strains of human and bovine infections, 14 showed neither reduction nor growth in the presence of a 1.5000 concentration of methylene blue. Of the 38 strains from dairy products 32 reduced the dye and grew in this concentration.
The results with the non-hemolytic streptococci indicate that the saprophytic strains have a greater tolerance for the dye than have the strains of parasitic origin.
The significance of these results in the differentiation of streptococci of human, bovine and dairy origin is discussed.